The present invention relates in general to pulse oximetry and, in particular, to detecting and responding to unreliable signal conditions such as conditions associated with a sensor that is detached or misaligned or otherwise associated with a sensor that is not operating in a reliable measurement mode to measure a value related to oxygen saturation of arterial blood (collectively, xe2x80x9csensor off conditionsxe2x80x9d).
In the field of photoplethysmography, light signals corresponding with two or more different centered wavelengths may be employed to non-invasively determine various blood analyte values. By way of example, blood oxygen saturation (SpO2) levels of a patient""s arterial blood are monitored in pulse oximeters by measuring the absorption of oxyhemoglobin and reduced hemoglobin using red and infrared light signals. The measured absorption data allows for the calculation of the relative concentrations of reduced hemoglobin and oxyhemoglobin, and therefore SPO2 levels, because reduced hemoglobin absorbs more light than oxyhemoglobin in the red band and oxyhemoglobin absorbs more light than reduced hemoglobin in the infrared band, and because the absorption relationship of the two analytes and the red and infrared bands is known.
To obtain absorption data, pulse oximeters include a probe that is releasably attached to a patient""s appendage (e.g., finger, earlobe or the nasal septum). The probe directs red and infrared light signals to the appendage, or tissue-under-test. The light signals are provided by one or more sources which are typically disposed in the probe. A portion of the light signals is absorbed by the tissue-under-test and the intensity of the light transmitted through or reflected by the tissue-under-test (i.e., the modulated signal) is detected, usually by at least one detector that may also be located in the probe. The intensity of an output signal from the detector(s) is utilized to compute SPO2 levels, most typically via a processor located in a patient monitor interconnected to the probe.
As noted above, the probe is releasably attached to the patient""s appendage. In this regard, it is useful for the pulse oximeter to identify sensor off conditions such that the oximeter can provide an appropriate indication, e.g., by blanking or dashing the display and/or providing an alarm. Identifying such sensor off conditions is a particular concern in certain pulse oximetry environments. For example, certain disposable or single-use sensors are designed to wrap around a finger or other appendage of a patient and may be subject to sensor off conditions if not properly attached or otherwise due to patient movement. Similarly, pulse oximetry sensors used on pre-mature babies and other infants are difficult to keep properly attached/aligned because of limitations on attachment due to skin sensitivity concerns and also because of erratic movements of the infant. Even in other pulse oximetry environments, the sensor may become detached or misaligned.
The conventional approach to identifying such conditions is to analyze the signal output by the sensor. If the analyzed signal is determined to be inconsistent with an expected photoplethysmographic signal, a sensor off condition may be deemed to exist. However, such conditions are difficult to accurately identify for a number of reasons. First, the photoplcthysmographic waveform or xe2x80x9cplethxe2x80x9d is generally small in magnitude and can vary substantially from time to time and patient to patient. Accordingly, the pleth is difficult to characterize and is difficult to distinguish from various sources of noise. Moreover, if a detached sensor is swinging, vibrating or subject to other periodic motion, the resulting detector signal during a detector off condition may be modulated in a manner that mimics, to an extent, a pleth. Consequently, designers have devoted significant. effort to accurately detecting sensor off conditions.
In certain pulse oximetry products marketed by Datex-Ohmeda, a sensor off condition associated with a detached sensor has been detected based on the use of a correlation coefficient. Generally, a correlation coefficient is used in many fields to analyze how well two signals correlate to one another in terms of waveform and other signal characteristics. In the context of pulse oximetry, a correlation coefficient can be used to compare how well the signal from one channel (e.g., the red channel) compares to the signal from another other channel (e.g., the infrared channel). In the case of a pleth, a high level of correlation is anticipated. The noted Datex-Ohmeda products therefore monitor the correlation coefficient over time to obtain an indication of signal quality. If a low level of correlation is sustained over a time window a sensor off condition is deemed to exist. Specifically, if the mean of the correlation coefficient squared falls below a predetermined threshold continuously for a period of twelve seconds, a sensor off condition is detected.
The present invention relates to an improved system for detecting sensor off conditions. It has been observed that the signal quality of a pulse oximeter has a greater volatility during sensor off conditions than under conditions where a pleth is reliably measured. That is, sensor off conditions can be detected not only based on low levels of detected signal quality but, additionally or alternatively, based on changes in the level of detected signal quality. Indeed, it has been verified that. certain sensor off conditions can be more reliably identified using calculations involving signal quality volatility than using conventional techniques. The present invention thus allows for improved sensor off detection and response.
In accordance with one aspect of the present invention, a sensor off condition is detected based on a variance in detected signal quality. The associated method involves defining a measure of signal quality, monitoring the measure of signal quality over time, calculating a value related to variance of the signal quality measure, and identifying a sensor off condition based on the variance value. The measure of signal quality may be any suitable measure for quantifying the likelihood that a detected signal is a pleth. For example, the step of monitoring the signal quality may involve identifying two channels of the detector signal (e.g., red and infrared), determining a correlation coefficient based the signals of the two channels and monitoring the correlation coefficient with respect a moving window of a predetermined temporal length (e.g., several seconds).
Various mathematical models may be used to calculate a value related to variance or volatility of the signal quality measure. Such models may determine variance based on a comparison of pairs of data points, multiple data points or based on a mathematical/statistical analysis of all data within a selected window of data. For example, one such indication of variance is provided by computing a standard deviation of the signal quality over a time window. It will be appreciated that the time window for consideration of the variance value may be different than a time window for consideration of the signal quality measure. That is, the windows may be different in length and/or translated in time relative to one another. The calculated variance may be used alone or in combination with other parameters to identify a sensor off condition. For example, the sensor off condition may be based on comparing the variance value or a series of such values (or other values calculated therefrom) to a threshold.
In accordance with another aspect of the present invention, two different signal quality related values are used to identify a sensor off condition. As above, a measure of signal quality is defined and monitored. First and second values related to the signal quality measure are then calculated and used to identify a sensor off condition. In one implementation, the first value relates to a magnitude of the signal quality measure (e.g., an average of the measure over a time or data window) and the second value relates to a variance of the signal quality measure (e.g., a standard deviation of the measure over the same or a different time or data window). A ratio of the first and second values may be calculated and compared to a threshold or thresholds to identify a sensor off condition. Other system parameters may be considered in establishing or selecting a threshold level.
According to further aspect of the present invention, a detector off condition is identified based on a volatility of a signal quality of a filtered detector signal. The detector signal is first filtered, for example, to reduce the effects of noise or to otherwise reduce or eliminate components extraneous to the pleth of interest. For example, Blackman filter coefficients may be applied to a series of values of a channel signal data window to smooth out certain noise effects. The filtered signal is used to perform a volatility measurement. In this regard, a correlation coefficient may be computed relative to two channels of the filtered signal, and the correlation coefficient may be monitored over a moving window to calculate variance values for the correlation coefficient (e.g., standard deviation values). The resulting variance values may be used alone or in combination with other parameter values to identify a sensor off condition. It has been found that improved detection of sensor off conditions can be achieved by using such a variance value calculated based on a filtered signal.